A capacitive touch device detects a touch point by sensing the capacitance variation of a capacitive touch sensor. For example, referring to FIG. 1, a two-dimensional capacitive touch sensor 10 has traces X1-X9 in the X direction and traces Y1-Y8 in the Y direction, where the traces X1-X9 and Y1-Y8 are not electrically connected to each other, in which arrangement each of the traces X1-X9 and Y1-Y8 has a self capacitance, and each intersection of the traces X1-X9 and Y1-Y8 has a mutual capacitance between the two traces involving the intersection. When a finger 12 touches the two-dimensional touch sensor 10, the self capacitances and the mutual capacitance sensed from the traces of the touch point have capacitance variations. The sensing schemes for capacitive touch devices are conventionally classified into all point scan and projection scan. Sensing methods based on all point scan sense the variations of the mutual capacitances at all trace intersections during a sensing period of a frame, while sensing methods based on projection scan sense the variations of the self capacitances of all traces X1-X9 and Y1-Y8 during a sensing period of a frame and then generate the intersecting points by projection from the X and Y directions. In sensing the variation of a self capacitance, taking the trace Y7 as an example, an excitation signal Tx is applied to the trace Y7, and a sensed signal Rx is received from the same trace Y7. The sensed value generated from the sensed signal Rx is processed, for example, compared to a base self capacitance or a threshold, to extract the variation of the self capacitance of the trace Y7. When a finger 12 touches the trace Y7, the capacitance of the finger 12 to the ground plane is parallel connected to the self capacitance of the trace Y7, and thus the sensed value from the trace Y7 is different from that obtained when the trace Y7 is not touched, from which it can identify whether the trace Y7 is touched. In sensing the variation of a mutual capacitance, taking the traces X8 and Y7 for example, an excitation signal Tx is applied to the trace Y7 (or X8), and a sensed signal Rx is received from the corresponding trace X8 (or Y7). The sensed value generated from the sensed signal Rx is processed, for example, compared to a base mutual capacitance or a threshold, to extract the variation of the mutual capacitance between the traces X8 and Y7. When a finger 12 touches the intersection of the traces X8 and Y7, the capacitance of the finger 12 to the ground plane reduces the sensed mutual capacitance, by which it can identify the intersection of the traces X8 and Y7 is touched.
When sensing a capacitive touch sensor, noise interference can bring errors to the sensed value, causing a point that is not touched to be mistaken as a touch point, or causing the reported coordinates inaccurate, which in turn leads to misoperation. For example, referring to FIG. 1, when a finger 12 touches a point between the traces X7, X8 and Y7, Y8, the coordinates of the touch point can be calculated from either the variations of the mutual capacitances between the traces X7 and X8 and between the traces Y7 and Y8 or the variations of the self capacitances of the trace X7, X8, Y7 and Y8. However, if the sensed value is error due to noise interference, the calculated coordinates will be deviated from the actual position of the finger 12. If, due to noise interference, the sensed value of the variation of the mutual capacitance between the traces X3 and Y7 or the sensed values of the variations of the self capacitances of the traces X3 and Y7 exceeds the threshold that is set by the sensing circuit, the sensing circuit will identify the intersection 14 of the traces X3 and Y7 as another touch point. For eliminating mistakes caused by noise interference, an approach is to use a median filter for filtering signals, yet this can reduce the sensitivity. Another approach is to raise a threshold of the algorithm, but this can make the capacitive touch sensor less supportive to hardware. Yet another approach is to further check the sensed values of a frame each time the scan for the frame is completed, and if the sensing for the frame is identified that the noise interference level exceeds the preset value, discard the sensed values of the frame and sense for the frame again until acceptable sensed values are obtained. However, this approach is passive and lowers the frame rate that will slower the response of the capacitive touch device and cause users operate not smoothly.
Conventional solutions all compromise other parameters, and therefore it is desired a sensing method and apparatus for suppressing noise interference without compromising other parameters.